Journal of Interdisciplinary Dentistry

ORIGINAL ARTICLE
Year
: 2022  |  Volume : 12  |  Issue : 3  |  Page : 83--89

A comparative evaluation of shear bond strength of direct composite laminates using different surface treatments: An in vitro study


Jyoti Gupta, Abhinav Agarwal, Manesh Lahori 
 Department of Prosthodontics, KD Dental College and Hospital, Mathura, Uttar Pradesh, India

Correspondence Address:
Dr. Jyoti Gupta
KD Dental College and Hospital, National Highway #2, P.O. – Chhatikara, Mathura - 281 006, Uttar Pradesh
India

Abstract

Background: In dentistry re-establishing, a lost dental esthetic appearance is one of the most important topics. Advances in modern dental materials provide patients with a choice of natural-looking laminates as esthetic restorations. Aim: This study aims to evaluate and compare shear bond strength of direct composite laminate veneer using different surface treatments. Setting and Design: A total of 60 permanent maxillary central incisors were selected for the study. The extracted teeth were thoroughly cleaned and stored in 3% hydrogen peroxide at room temperature for 24 h and then stored in normal saline. Materials and Methods: The facial surfaces of the teeth were prepared to accommodate laminates of equal thickness and were randomly divided into four groups which were allocated to receive different surface treatments. Group A and B included total-etch adhesive system with etching time 15 s and 60 s, respectively, Group C included a self-etch adhesive system with two components and Group D included self-etch adhesive system with single component. After these surface treatments, direct composite laminate veneers were placed on the teeth samples. Later, thermocycling process was carried and final testing was done using universal testing machine. Statistical Analysis: Results were compared with 1-way ANOVA and post hoc Tukey HSD test. Results: The mean shear bond strength was found to be highest for the total-etch method with 60s etching time followed by etching for 15s and the lowest was found to be of self-etch adhesive comprising of a single component. Conclusions: The study concluded that the mean shear bond strength of direct composite laminate veneers using total-etch adhesive system with 60 s etching time was found to be significantly higher compared to other surface treatment methods used in the study.



How to cite this article:
Gupta J, Agarwal A, Lahori M. A comparative evaluation of shear bond strength of direct composite laminates using different surface treatments: An in vitro study.J Interdiscip Dentistry 2022;12:83-89


How to cite this URL:
Gupta J, Agarwal A, Lahori M. A comparative evaluation of shear bond strength of direct composite laminates using different surface treatments: An in vitro study. J Interdiscip Dentistry [serial online] 2022 [cited 2023 Feb 2 ];12:83-89
Available from: https://www.jidonline.com/text.asp?2022/12/3/83/365608


Full Text



 Clinical Relevance to Interdisciplinary Dentistry



Advances in dentistry provide patients with a choice of natural-looking laminates as esthetic restorations. For its longevity, a vital importance is attributed to the strength and durability of the adhesive system.

 Introduction



The beauty of smile and esthetics of anterior teeth are considered synonymous. In dentistry re-establishing, a lost dental esthetic appearance is one of the most important topics. Any change in morphology, color, and shade in relation to anterior teeth requires the careful intervention of prosthodontic treatment. The technique preferred frequently is to cover the teeth with dental crowns. However, excessive preparations of teeth and damages to surrounding tissues, such as gingiva, are some major disadvantages of crowns.[1] Recently, laminate restorations have been used as a more esthetic and more conservative treatment option in dentistry.

Advances in modern dental materials provide patients with a choice of natural-looking laminates as esthetic restorations. These may be directly fabricated composite laminate veneers or indirectly fabricated composite or porcelain laminates.[2] Indirect porcelain laminate has high resistance against attrition, fractures, and discolorations[3] compared to that composite laminates. However, it also includes several disadvantages such as long chair time, higher cost, and use of an adhesive cementing system.

Direct composite laminate veneers are placed directly on prepared tooth surfaces intraorally which requires minimal tooth preparation, is more economic, and does not require an additional adhesive cementing system. They are easy to polish and have better marginal adaptation compared to indirect laminate veneer restorations.[3] Tooth discolorations, rotated teeth, coronal fractures, congenital or acquired malformations, diastemas, discolored restorations, palatally positioned teeth, absence of lateral incisors, abrasions, and erosions are the main indications for direct composite laminate restorations.[1]

The initial preparation for laminate is basically done using diamond burs of preestablished depths which have standardized measures for a reduction on the labial surface. Preparation should be completely in enamel to maintain an optimal bond with laminates and decrease stress on composites. Depending on location, usually 0.3–0.7 mm reduction is necessary to remove the aprismatic and hypermineralized enamel top surface, which can be resistant to acid etching.[4]

For the longevity of the composite laminate veneers, a vital importance is attributed to the strength and durability of the adhesive system. Adhesive systems are basically classified as either etch and rinse or self-rinse systems. The etch and rinse systems imply phosphoric acid etching step, followed by water rinsing and drying of the surfaces before the application of the adhesive system. The main aim of etching enamel is to clean the surface pellicles in uncut enamel, to remove smear layer in cut enamel and to partially dissolve crystalline minerals to form retentive patterns.[5] Acid etching increases resin-enamel bonding strength by creating micro-porosities in the etched surface which help in resin infiltration and formation of resin tags.[5],[6] Clinicians face several challenges during the adhesive pretreatment procedure and may frequently be confronted with the issue of accidentally over-etched enamel. Therefore, the study also aims to assess the influence of over-etching time on the bond strength of a universal adhesive in contrast to solely increased acid etching times followed by an adhesive application time as per the manufacturer's instructions.

The self-etch system, however, relies on self-etching acidic monomers instead of phosphoric acid to rather integrate the smear layer and simultaneously demineralize and infiltrate dental hard tissue.[7],[8] Each bonding approach has different generations of bonding agents that can be applied together in a single bottle or separately depending on the adhesive system. According to that, the etch-and-rinse adhesive can be done using three steps or two steps, and the self-etch system can be done using either two steps or one step.[9]

The shear bond strength of direct composite laminate veneers to teeth depends on different adhesive approaches and also the duration of etching. Therefore, the study aims to evaluate shear bond strength of direct composite laminate veneers using different surface treatments.

 Materials and Methods



A total number of 60 permanent maxillary central incisors, noncarious, without any gross defects, and not endodontically treated were selected for this study. The extracted teeth were thoroughly cleaned and stored in 3% Hydrogen peroxide (H2O2) at room temperature for 24h. Hydrogen peroxide solution acts as a disinfectant and remove all the microorganism present on the tooth surface. According to literature, concentration above 3% H2O2 damage the microhardness of enamel and dentin so in this study, 3% H2O2 was used. A putty index was prepared using wax block of dimension 1.5 cm × 1.5 cm which was used as a mold for embedding teeth. After that, each tooth was placed into a mold and embedded in auto-polymerizing methyl methacrylate resin. The teeth samples were embedded just below the cementoenamel junction level.

The facial surfaces of the teeth were prepared to accommodate laminates of equal thickness. Self-limiting depth-cutting disks of 0.3 mm and 0.5 mm were used to make depth grooves, 0.3 mm on cervical third and 0.5 mm on middle and incisal third [Figure 1]. With the help of permanent marker marking was done on the depth grooves. Preparation was continued till the marking became invisible from the middle 3rd region using chamfer diamond burs. The prepared putty index was used to measure the thickness of preparation. All tooth preparations were completed entirely in enamel, without sharp line angles.{Figure 1}

A total of 60 prepared teeth samples were randomly divided into four groups. Each group comprised 15 teeth and was allocated to receive different surface treatments.

Group A

Prepared samples were cleaned and dried. Etching was done with 37% phosphoric acid for 15s (recommended etching time by manufacturer) followed by thoroughly rinsing with water, ensuring that all the etchant was removed. After that, the sample was air dried using a 3-way syringe followed by the application of 5th generation bonding agent (3M ESPE universal adhesive-total-etch adhesive system) using applicator and cured using a light cure unit for 10 s.

Group B

Prepared samples were cleaned and dried. Etching was done with 37% phosphoric acid for 60 s (extended etching time) followed by thoroughly rinsing with water and dried properly. The 5th generation bonding agent was applied and cured for 10 s.

Group C

Prepared samples were cleaned and dried. Self-etch adhesive (Clearfil SE) comprising of two components was used as bonding agent. First self-etch primer was applied and left for 20 s then the adhesive was applied directly without cleaning primer and light cured for 10 s. Instructions were followed as mentioned in manual.

Group D

At first, the prepared samples were cleaned and dried properly. Self-etch adhesive (GC Solare) comprising one component was used as a bonding agent. One component bonding agent was applied directly to prepared surface and light cured for 10s.

After the different methods of surface treatments, the placement of direct composite laminate veneers was done on the prepared surface followed by light curing for 20 s. The equal thickness of laminates was maintained and checked using the putty index prepared before. After the placement of direct composite laminate veneers, the samples were stored in distilled water at 37°C for 24h. After that, thermocycling process was carried out to simulate oral condition. Thermocycling chamber used here consists of a temperature controller and specimen transfer mechanism. Thus, thermocycling was carried to cold and hot sections alternatively for 5000 cycles between 5°C and 55°C with a dwell time of 60 s. The shear bond strength test was performed using universal testing machine. The acrylic mold was mounted in the lower member and the upper member had the mono-bevel chisel with a crosshead. The force was applied at 90° to the long axis of the tooth on incisal edge vertically at a cross-head speed of 0.5 mm/min [Figure 2] until detachment or fracture occurred. The maximum load was recorded in Newtons for each sample and shear bond strength was calculated in megapascals by dividing the load with surface area of the tooth after the detachment of laminates. A total of 60 test samples were tested in an identical manner and the shear bond strengths were tabulated for statistical analysis.{Figure 2}

 Results



The mean shear bond strength was found to be highest for Group B–15.35 MPa followed by Group A–14.09 MPa. The mean shear bond strength of Group C–13.71 MPa was found to be slightly less than Group A (15.35 MPa). The lowest shear bond strength was found to be of Group D (11.84 Mpa) [Table 1] and [Graph 1].{Table 1}[INLINE:1]

In this study, one-way ANOVA was used to determine the statistical difference in shear bond strength within four groups (A, B, C and D). There was a statistical difference between four groups (F = 65.721, P = 0.000) [Table 2]. Since significant differences were determined using One-way ANOVA, the results were further analyzed using the Tukey – HSD test at a significant level of 5%. This was done to determine where the differences between groups and within each group lie. According to this multiple comparison test, there was a significant difference (P = 0.000) between groups A and B, A and D, B and C, B and D, C and D whereas, there were nonsignificant differences (P = 0.451) between group A and C [Table 3].{Table 2}{Table 3}

 Discussion



With the advancement in dentistry, various materials and methods are introduced with aims to achieve esthetics and successful dental rehabilitation with minimal invasiveness. Direct composite laminate veneers are considered good treatment option to restore the esthetics of anterior teeth. For the longevity of the composite laminate, a vital importance is attributed to the strength and durability of the adhesive system.

The most common method of adhesion followed since decades is total etch technique or etch-and-rinse method. Ideally, adhesives should reach the bottom of the demineralized layer and entirely infiltrate micro-porosities created to prevent nanoleakage and decrease of bond strength.[9] The etching of enamel with acid removes the smear layer and increases resin-enamel bonding strength by creating microporosities in the etched surface which help in resin infiltration and the formation of resin tags.[5],[6] However, this method involves many steps and requires more chair time.[10],[11]

The current trend in adhesive dentistry is focused on mainly using systems that are simple to use and have minimal chair time. The newest adhesives have replaced the total-etch technique with one- or two-step self-etch technique that combine the etching step with monomer infiltration to a shallow hybrid layer.[12] Acidic monomers in the primers of self-etching adhesives etch enamel and modify or dissolve the smear layer. Studies have shown that the resultant enamel surface is not etched to the same degree with self-etch adhesive systems when compared with traditional phosphoric acid conditioning.[13],[14],[15] However, clinical success of this method is still questionable.

In this study, various methods of adhesion were evaluated to measure the most effective way of bonding of direct composite laminate veneers with human teeth. The current study indicates the mean shear bond strength of total etch system with recommended and extended etching time, i.e., 15 s and 60 s, respectively, was significantly greater (P < 0.05) than that produced by the self-etch systems (one component and two components system). This finding is in line with a previous study that showed etch and rinse systems resulted in higher bond strengths than self-etch systems.[16],[17],[18],[19] A study conducted by Barkmeier et al.[20] also showed a progressive increase in mean enamel surface roughness (Ra) created by applying a 37% phosphoric acid gel for 15 s and 60 s than that produced by the four self-etch systems. Legler et al.[21] also showed a progressive increase in enamel surface roughness using a 37% phosphoric acid solution for 15 s, 30 s, and 60 s on 600 grit flat ground surfaces. However, some studies concluded that self-etching adhesive systems produced higher bond strength than conventional total-etch systems, especially the all-in-one system, which produced the highest bond strength.[22] Whereas, some studies stated that self-etch system and total-etch system showed comparable dentin bond strength.[23],[24]

This study showed that the mean bond strength of direct composite laminate veneers is more for extended etching time (60 s) than that of normally recommended etching time (15 s) which may be due to increase in demineralization depth, micro-porosities, and surface roughness.[25],[26] However, results of some studies indicate that there was no any statistically significant difference in bond strength between different etching time.[9],[20],[27] This is also in line with a study by Paul et al.,[28] in which no significant differences in bond strength were found regardless of a performed over-etching up to 60 s but constant adhesive infiltration time. While contrast to this study, some studies have shown that extending this time, produces morphological changes that may decrease the ability of adhesive resin to bond to the demineralized surface.[29],[30],[31]

This study also demonstrated that there was no significant difference (P = 0.451) between the mean bond strength of two-step self-etch and total-etch method with etching time of 15s. However, total-etch method showed slightly higher mean shear bond strength compared to two-step self-etch method. According to Kerby et al., self-etch adhesives produced thinner and shorter resin tags than those produced by phosphoric acid etching and thus resulting in inferior bond strength as compared to total-etch adhesive systems.[32] Self-etching adhesive systems rely on acidic monomers to simultaneously demineralize and infiltrate enamel. This acidity must be neutralized by the mineral content of the tooth structure, to allow complete polymerization of the adhesive film.[16]

Most single-step self-etch adhesives contain hydroxyethyl methacrylate, which can polymerize in the presence of water to form microporous hydrogel with pore size ranging from 10 to 100 nm.[16] Differential water movement across the cured adhesive layer may occur in the presence of increased concentration of dissolved inorganic ions. The concentration difference may establish an osmotic pressure gradient, causing water movement from a region of low solute concentration to a region of high solute concentration. This may cause water blisters, which act as weak spots along the adhesive interface.[33]

The lowest mean shear bond strength in this study was found to be of the single component self-etch method. Application of only the single-step self-etch adhesive resulted in a shallow etching pattern that might be the result of limited penetration of the adhesive into the enamel micro porosities or a result of calcium precipitation on the enamel surface masking the etch pattern, then interfering with resin penetration.[34],[35] This finding is in line with a previous studies that showed one component self-etch system has lower bond strengths compared to that of two components self-etch system.[16],[36]

With single-step self-etch systems, the possibility of monomer not infiltrating demineralized enamel is reduced. Since the etching effect on an enamel surface is an ionic process, water is indispensable for the ionization of the acidic functional monomers. The amount of water to be included in the single-step self-etch adhesive is critical. It should be large enough for ionization of the acidic functional monomers, yet sufficiently small to preserve a high amount of resin monomer concentration.[37] Even if complete penetration into the demineralized enamel can be achieved, the degree of conversion of resin monomers might be impacted by the presence of the remaining water in the adhesive.

The achievement of strong micromechanical bonding of composite laminates depends on the depth of monomer penetration into demineralized enamel. Self-etching adhesives are capable of penetrating the aqueous channels formed between the smear layer particles, widening these channels, and interacting at the top of the underlying dentin. These agents offer a simpler clinical application than total-etch systems because they are capable of conditioning the tooth surface and simultaneously preparing it for adhesion. However, they provide lower bond strength than total-etch systems because of their semi-permeability, incorporation of smear layer, shorter resin tag formation, residual acidity, and hydrolytic instability.

 Conclusions



The study concluded that the mean shear bond strength of direct composite laminates using total-etch adhesive system with an etching time 60 s was found to be significantly higher compared to other surface treatment methods used in the study. Although the self-etch adhesive system offers a simpler clinical application than that of total-etch adhesive system, the result of this study showed better bond strength using total-etch adhesive system.

Further studies are recommended to evaluate the effect of different surface treatments clinically to achieve the results obtained with the present study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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